1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with the generation of tones corresponding to selected autocorrelation functions.
2. Description of the Prior Art
The current trend toward digital tone generations systems is primarily motivated by the quest for a commercially viable musical instrument which can produce good tone characteristics and which can be implemented at a commercially acceptable cost. The prior art in digital tone generators can be broadly distinguished into three generator classes.
The first generator class is one in which the tone generation system depends upon a stored library of musical waveshapes. Each stored waveshape corresponds to a tone control, or stop, located on the console of the musical instrument. The stored waveshapes are read out of memory at an address advance rate of f=NF where N is the number of data points per period of the waveshape and F is the fundamental pitch of the generated musical tone. A tone generator of the first generator class is described in U.S. Pat. No. 3,515,792 entitled "Digital Organ."
A second generator class is one that is called by the generic name of "harmonic synthesis." Tone generators of the second generator class are operated by implementing a discrete Fourier transform to compute points for a musical waveshape using a stored library of harmonic coefficients. Subsets of the stored harmonic coefficients are selected by actuating tone control switches. Tone generators belonging to the second generator class are described in U.S. Pat. No 3,809,786 entitled "Computor Organ" and in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer."
A third generator class is one in which a number of harmonic components are obtained by frequency modulating a sinusoid function at the fundamental frequency of the generated musical tone. Various tones are obtained by selection of the modulation indices as well as by varying the frequency and waveshape of the frequency modulating signal. A tone generator of the third generator class is described in U.S. Pat. No. 4,175,464 entitled "Musical Tone Generator With Time Variant Overtones."